Background: The thromboembolic risk of atrial flutter (AFL) is not well defined. On the other hand, in atrial fibrillation (AF), the echocardiographic demonstration of thrombus or spontaneous echo contrast in the left atria or its appendage, a lower flow velocity in the left atrial appendage, and its reduced mobility, are well known risk factors of thromboembolism. Aim: To study the incidence of these echocardiographic risk factors in patients with AFL. Material and methods: We prospectively studied 50 consecutive patients with AFL comparing them with two groups of patients with a well known increased risk of thromboembolism: 54 patients with AF and 24 patients with sinus rhythm and severe mitral stenosis (RSEMS). The group of patients with AFL was also compared with a control group of 27 patients with sinus rhythm and no increased risk of thromboembolism. In each group, we studied the presence of thrombi and spontaneous echo contrast in the left atria and left atrial appendage, emptying velocity (Vel A), filling flow (Vel B) and motility of the left atrial appendage and left atrial dimensions. Results: When compared with control patients, AFL subjects had a higher incidence of spontaneous echo contrast in the left atria and left atrial appendage (11 and 42% respectively, p<0.05); slower flow velocity in the left atrial appendage (Vel A 69.25 ± 25 and 41 ± 19 cm/s respectively, Vel B 55 ± 16 and 46 ± 20 cm/s respectively, p<0.05); lower atrial appendage wall motility (4 and 84% respectively, p<0.001) and a larger left atrium (40 ± 10 and 45 ± 0.6 mm respectively, p<0.05). Patients with AFL had a lower incidence of echocardiographic abnormalities than subjects with AF or RSEMS. Thrombi were found in 2 patients with AFL, 12 patients with AF, 4 patients with RSEMS and in no control patient. Conclusions: In AFL, there are echocardiographic markers of increased thromboembolic risk in comparison with a control group. Nevertheless, the incidence of these factors is lower than in patients with AF or with RSEMS (Rev Méd Chile 2000; 128: 1327-34).